There is a demand in the closed die forging industry for forging dies having better die life in order to increase production and reduce unit costs. In view of these demands, the early failure of dies by heat cracking, which is characterized by the appearance of a plurality of fine cracks in the die surface, and washing, which is the wearing away of the die so that the die impression goes oversize, has spurred the search for better implements. In hot forging operations for example the die surface may attain a working temperature of from about 600 to 1,100 degrees F. when operating at room temperature, since it is exposed to intermittent contact with steel or other alloys which are at a temperature of around 2,200 degrees F. Other materials, such as titanium alloys, stainless steel, aluminum alloys, high temperature alloys, and Inconel may be forged at the same or lower temperatures. Some materials, such as the titanium alloys, additionally raise corrosion problems, and sometimes the dies are used to forge even higher temperature stock, such as tungsten alloys.
Forging die steel producers have long recognized that if the lower critical temperature of the die material can be elevated while all the other desirable operating characteristics are retained, better die life can be expected. Die steel which, in operation, is heated to a point much above the critical temperature (as by over exposure with the work piece, such as would occur when the work piece is locked in the dies) will reharden on cooling and may crack. It is also well known in the art that the tempering temperature of the steel is related to the critical temperature in the sense that for a given hardness, the higher the lower critical temperature of the steel, the higher the tempering temperature as a general rule. This factor is of importance in this industry because die steels intended for use in closed die forging are conventionally supplied to a specified hardness range, and therefore the higher the tempering temperature for a given hardness, the higher the temperature the die steel can withstand without the danger of softening or cracking.
It will be understood unless the context indicates otherwise that the term "lower critical temperature" is intended to be interpreted in its usual sense; that is, as defining the temperature at which a phase transformation from a ferritic or body-centered cubic structure to an austenitic or face-centered cubic structure occurs. As is well known, transformation of the steel will upset previous heat treatments.
In U.S. Pat. No. 3,519,499, the importance of elevating the lower critical temperature is discussed, and a broad range of elements is set out which have the effect of achieving this important objective, which represents a substantial advance in the hot work industry.